1. Field of the Invention
The present invention generally relates to a method for controlling a cooking operation of a microwave oven.
2. Description of the Prior Art
The following description relates to the prior art. Firstly, Japanese Laid-open Patent No. Sho 63-254326 published Oct. 21, 1988 discloses an apparatus for displaying an error when a controller senses that no food has been placed on a tray in a cooking chamber. This apparatus stores a tray weight in the controller, and thus any sensed weight that is above the stored tray weight when the tray acts upon a weight sensing mechanism represents the weight of the food. Also, if the rotary tray does not properly act upon the weight sensing mechanism, the microcomputer displays an error.
U.S. Pat. No. 4,615,405 and Japanese Laid-Open Patent Publication 96-320123 disclose microwave ovens having a weight sensor for sensing the weight of food on a tray of a microwave oven. The U.S. patent discloses that the cooking time period is calculated as a function of the sensed food weight.
Japanese Utility Model Publication (unexamined) No. 03282124 (published Dec. 12, 1991 discloses a microwave oven that includes a spin chuck table, a rotary tray located on the spin chuck table, and a tray elevating device which raises the tray to a prescribed height. When the tray elevating device goes into action, the tray is elevated after being disjoined from the spin chuck table. The tray then rotates, and microwaves generated by a magnetron are uniformly transferred, even to the bottom of the tray.
A conventional microwave oven depicted in FIG. 5 includes: a metallic cabinet 10; a cooking chamber 11; a magnetron (not illustrated) which emits high-frequency microwaves to the cooking chamber 11; a high voltage transformer 13 which supplies high voltage to the magnetron; an electric resistance heater 17 that is mounted on the upper porion of the cooking chamber 11 to cook foodstuffs in the cooking chamber 11 by radiant heat and convective heat; and a food tray 12 which is provided on the inner bottom of the cooking chamber, and is designed to be elevated and to be rotated about a vertical axis. In addition, a door (not illustrated) is installed on the front side of the cooking chamber 11, to open/close the cooking chamber, a cooling fan for cooling the above components is installed to the back side of the electrical component compartment 14. A terminal is formed at an end of the heater 17 so that power is supplied to the heater 17. The heater can be rotated between horizontal and vertical states. In this case, a position of the heater can be changed by the user, and various cooking operations are possible according to the position of the heater.
The above microwave oven also includes a shaft 31 having an upper end connected with the bottom of the tray 12 and a lower end extending downward to the outside of the cooking chamber 11; an elevation guide member 34 positioned under the shaft 31 to elevate the shaft 31; an elevating motor 33 that reciprocates the elevation guide member right and left; a motor 32 that rotates the shaft 31 by means of a gear 32a; and a weight sensing mechanism 35 that is provided under the elevation guide member 34 to measure the weight of a foodstuff on the tray 12.
The following description relates to the operation of the above microwave oven.
If microwaves of about 2,450 MHz in frequency are applied to the food, molecules of the food contained in a metallic case are motivated by microwave energy so the food emits heat. A microwave oven is an oven that uses microwave heating for fast cooking of meat and other foods.
When the magnetron emits microwaves of about 2,450 MHz to the metallic cabinet 10, molecules of the food are each positively and negatively charged. That is, one side of each food molecule is negatively charged by the positive electrons of the electric field created by the microwaves, and the other side of each food molecule is positively charged by the negative electrons of the electric field. Since the electric field's polarity is changed about 2.4 billion, 5 thousand times per second, the food's molecules collide with each other to thereby create friction heat so that the food is heated up.
Assuming that the weight sensing mechanism 45 determines that food has been placed on the tray, the oven can perform fast cooking of foods by microwave energy or heat, while rotating the tray or moving it up or down. The tray is elevated to a predetermined height simultaneously with a starting operation. When the elevation of the tray 12 is completed, the tray is rotated and foodstuff on the tray 12 is cooked by the microwave energy generated from the magnetron.
In the meantime, if a use selects a desired cooking mode and operates a key input panel (not illustrated), the magnetron and heater are controlled according to the selected cooking mode and cooking time. If the user selects cooking modes such as a warm mode, a thaw mode and the like, the magnetron is operated. If the user selects a grill mode, barbecue mode or pizza-baking mode, the heater is operated. In addition, the cooking can be achieved by using the magnetron and the heater simultaneously.
The food cooked in a conventional microwave oven may be fully cooked before the set cooking time has elapsed, especially when the user cooks a small quantity of food. As a result, the food can be excessively boiled or burned. Also, in that case, the magnetron and the heater will have been driven for an unnecessarily long time, thereby wasting energy.